1. Field of the Invention
The present invention relates to a switching power supply circuit of current-resonance type with the capability of improving the power factor.
2. Description of Related Art
Due to recently-developed switching elements that withstand relatively high voltages and large currents and operate at high frequencies, power supply units for producing intended d.c. voltages by rectifying the a.c. line voltage are mostly designed to be switching power supply units. Switching power supply units operating at higher switching frequencies enable the size reduction of the transformer and other circuit components, and are used as large-power d.c. to d.c. converters in power supply units of various kinds of electronic equipment.
Generally, when power of the a.c. line is rectified, the current flowing in the smoothing circuit has its waveform distorted, resulting unfavorably in the degradation of the power factor which indicates the efficiency of use of power. It is therefore required to suppress the harmonic waves derived from the distorted current waveform.
For dealing with this situation, the applicant of the present invention has previously proposed a switching power supply circuit with the capability of power factor improvement as shown in FIG. 13. This power supply circuit is a self-excited current-resonance power converter of half-bridge configuration.
In the figure, supplied a.c. line power AC is subjected to full-wave rectification by a bridge rectifier D.sub.1 made up of four diodes. The bridge rectifier D.sub.1 has its positive output terminal connected to the positive electrode of a smoothing capacitor Ci by way of a serial connection of a filter choke coil L.sub.N, a fast-recovery diode D.sub.2 and a choke coil CH, as shown. A filter capacitor C.sub.N is connected between the node of the filter choke coil L.sub.N and diode D.sub.2 and the positive electrode of the smoothing capacitor Ci, with the filter capacitor C.sub.N and filter choke coil L.sub.N in unison constituting an LC low-pass filter used in the normal operation.
The LC low-pass filter is intended to prevent the high-frequency switching noise from entering to the a.c. line. The fast-recovery diode D.sub.2 is used to cope with the current of the switching frequency flowing on the full-wave rectifier output line as will be explained later.
The capacitor C.sub.2 is used as a parallel resonant capacitor, which is connected in parallel to the choke coil CH as shown in the figure to form a parallel resonant circuit with it. The parallel resonant circuit has its resonant frequency set virtually equal to the resonant frequency of the switching power supply unit. The operation of the parallel resonant circuit will be explained later.
A half-bridge switching circuit is configured by a pair of switching elements Q.sub.1 and Q.sub.2, with their collectors and emitters being connected in cascade between the positive electrode of the smoothing capacitor Ci and the ground as shown in the figure. The switching elements Q.sub.1 and Q.sub.2 have their collectors and bases connected by respective starting resistors R.sub.S and have their bases and emitters connected by respective damper diodes D.sub.D.
The Q.sub.1 and Q.sub.2 have base resistors R.sub.B for adjusting their base drive currents connected in series to respective resonant capacitors C.sub.B, which constitute series resonant circuits for self-excited oscillation in unison with respective drive windings N.sub.B of a driving transformer PRT. The driving transformer PRT, which controls the switching frequency of the switching elements Q.sub.1 and Q.sub.2, has drive windings N.sub.B and a resonant current detecting winding N.sub.D, and it further has a control winding N.sub.C which is wound orthogonally to the windings N.sub.B and N.sub.D, thereby forming an orthogonal saturable reactor.
One drive winding N.sub.B has one end connected to the resistor R.sub.B and another end connected to the emitter of the switching element Q.sub.1. Another drive winding N.sub.B has one end grounded and another end connected to the resistor R.sub.B of the switching element Q.sub.2, and it has the opposite polarity of voltage relative to the former winding N.sub.B. The current detecting winding N.sub.D has its one end connected to the node of the emitter of Q.sub.1 and the collector of Q.sub.2 and another end connected through a series resonant capacitor C.sub.1 to one end of the primary winding N.sub.1 of an insulating transformer PIT.
The insulating transformer PIT, which transfers the switching output of the switching elements Q.sub.1 and Q.sub.2 to the secondary side, has a primary winding N.sub.1 which is connected at one end to the current detecting winding N.sub.D through a series resonant capacitor C.sub.1 and at another end to the node of the fast-recovery diode D.sub.2 and choke coil CH. The series resonant capacitor C.sub.1 and the inductance component of the insulating transformer PIT including the primary winding N.sub.1 form a resonant circuit permitting the switching power supply circuit to be of current-resonance type.
The insulating transformer PIT has a secondary winding N.sub.2, on which a voltage is induced in unison with the primary winding N.sub.1, and the induced voltage is rectified by a bridge rectifier D.sub.3 and smoothing capacitor C.sub.3 to produce a d.c. output voltage E.sub.o.
A control circuit 1 evaluates the difference of the d.c. output voltage E.sub.o from the reference voltage and amplifies the differential signal to produce a d.c. control output I.sub.C to be fed to the control winding N.sub.C of the driving transformer PRT.
In operation, when the a.c. line power is supplied to the switching power supply unit arranged as described above, the base currents are fed to the bases of the switching elements Q.sub.1 and Q.sub.2 by way of the starting resistors R.sub.S. In this case, one of Q.sub.1 and Q.sub.2, e.g., Q.sub.1, that turns on first causes another switching element, i.e., Q.sub.2, to be cut off. The resonant current flows from the switching element Q.sub.1 to the current detecting winding N.sub.D, to the capacitor C.sub.1, and to the primary winding N.sub.1. When the resonant current decreases to zero, the switching element Q.sub.2 turns on and the switching element Q.sub.1 turns off. The resonant current flows through the switching element Q.sub.2 in the direction opposite to that of the switching element Q.sub.1. In this manner, the switching elements Q.sub.1 and Q.sub.2 turn on alternately, and the self-excited switching operation starts.
The switching elements Q.sub.1 and Q.sub.2 turn on and off cyclically for the operational power on the smoothing capacitor Ci to feed a drive current with a waveform close to the resonant current waveform to the primary winding N.sub.1 of the insulating transformer PIT, and a.c. power is obtained on the secondary winding N.sub.2.
If the d.c. output voltage E.sub.o falls, the control circuit 1 controls the current of the control winding N.sub.C to lower the switching frequency (close to the resonant frequency) so as to increase the drive current of the primary winding N.sub.1, thereby stabilizing the output voltage E.sub.o (this is a method for switching frequency control).
In regard to the improvement of the power factor based on this circuit arrangement, the switching output corresponding to the resonant current flowing on the primary winding N.sub.1 of the insulating transformer PIT is superimposed directly on the rectified current of the a.c. line power flowing through the self-inductance Li of the winding Ni of the choke coil CH. Accordingly, the smoothing capacitor Ci is charged to the full-wave rectified voltage with the switching output voltage superimposed on it, and the terminal voltage of the capacitor Ci is lowered periodically by the superimposed switching output voltage. Consequently, a charging current flows during the period when the terminal voltage of the capacitor Ci is lower than the rectified voltage of the bridge rectifier, and the average a.c. input current is made close to the a.c. voltage waveform and the power factor is improved. The series resonant capacitor C.sub.1 is discharged back to the smoothing capacitor Ci by way of the primary winding N.sub.1 of the insulating transformer PIT during the period when the a.c. input current does not flow in.
The power supply circuit based on this scheme of power factor improvement has a smaller drive current of the insulating transformer PIT in small load condition, and accordingly the switching current flowing on the full-wave rectifier output line in response to the drive circuit is also small. Based on the charging current which is proportional to the load, it becomes possible to prevent the terminal voltage of the smoothing capacitor Ci from rising abnormally in small load condition and thereby improve the regulation. Even for a .+-.20% variation of the a.c. input voltage, for example, the variation of the rectified-and-smoothed voltage Vi is suppressed sufficiently, and the need to use switching elements and a smoothing capacitor with higher withstand voltages is eliminated.
The resonant capacitor C.sub.2 connected to the self-inductance Li of the choke coil CH is intended to suppress the switching voltage which is fed back to the rectify-smoothing line when the load of the switching power supply unit decreases, and it becomes possible to prevent the terminal voltage Ei of the smoothing capacitor Ci from rising in small load condition.
Namely, the power supply circuit of FIG. 13 operates to raise the switching frequency when the load power decreases, and in this case the switching voltage returning to the charging circuit is suppressed by the capacitor C.sub.2 and the rise of the terminal voltage is prevented. On the other hand, when the load power increases, the switching frequency falls close to the resonant frequency of the resonant circuit formed of the self-inductance Li and capacitor C.sub.2, thereby raising the feedback switching voltage. Accordingly, this power supply circuit has little variation of the terminal voltage of the smoothing capacitor caused by the variation of load, making the stabilization of d.c. output voltage Eo easy.
FIG. 14A and FIG. 14B show the filter choke coil L.sub.N and choke coil CH used in the power supply circuit of FIG. 13. The filter choke coil L.sub.N is arranged by winding a copper wire coated with polyurethane directly on a drum-shaped ferrite core D without using a bobbin as shown in FIG. 14A. The choke coil CH is made up of a pair of E-shaped ferrite cores facing each other, with a gap G being formed between the confronting central arms so that the magnetic flux does not leak to the outside of the choke coil as shown in FIG. 14B. A winding Ni which is a polyurethane-coated copper wire of 60-.mu.m diameter is wound on a bobbin (not shown) such that the inductance Li is not saturated in maximum load condition. Specifically, for the output power rating of 120 W of the d.c. output voltage E.sub.o, a choke coil CH of Li=15 .mu.H is arranged by winding a Litz wire at 60 .mu./80 bunch on the EE-16 core, or for the output power rating of 230 W, a choke coil CH of Li=100 .mu.H is arranged by winding a Litz wire at 60 .mu./130 bunch on the EE-25 core.